The invention relates to a data transfer interface and more particularly to a serial digital data interface system useful to disseminate digital data from a microprocessor or central station to any number of remote or slave stations as well as from any remote station to any other remote station or the microprocessor.
Digital computer technology, numerical machine and process control, large scale integrated circuits and microcircuit sophistication are combining to produce a revolution in the field of manufacturing and production information gathering and dissemination. The operations performed in a subsequent operation may be dependent upon the precise conditions under which the previous operation was performed. Production data required by a machine performing one phase of a manufacturing process may be stored in a central memory unit. Thus there is a necessity for a constant flow of data along a production line--data that must be produced, routed, verified and received throughout a complex system.
The requirement that any station be capable of receiving data from or transferring data to any or all other stations in the system is of some moment. One approach which requires a direct two-way interconnection between each station becomes intractable when the number of stations is great. Such an approach has often been abandoned in favor of a single data line along which the various stations are serially connected. A message is then preceded by an address code which identifies the station or stations to which it is addressed. The data is received and stored by every station in sequence. If the station is one to which the data were not addressed, the data would be retransmitted to the next, downstream station. Alternately, if the data were received by an addressed station, the data would be retained as well as retransmitted. Such a system has two shortcomings. First, since the data is transmitted ad seriatim through the stations, every station must be functioning in order for the data to be received by any downstream station. Secondly, a station cannot, without additional circuitry, transmit upstream, that is, it cannot transmit data to a station from which it receives data.
Optical couplers including light emitting diodes have proven to be a highly practical method of digital data transmission and an exemplary transmission system employing them is disclosed in U.S. Pat. No. 3,970,784. This patent discloses digital data stations both having receive and transmit capability linked by a single bi-directional data line. It is clear, however, that if this system were to be used with multiple stations, it would be necessary to utilize a separate pair of data lines to interconnect each station with every other station.